Internal combustion engines produce exhaust gases containing a variety of pollutants, including nitrogen oxides (“NOx”), carbon monoxide, and uncombusted hydrocarbons, which are the subject of governmental legislation. Increasingly stringent national and regional legislation has lowered the amount of pollutants that can be emitted from such diesel or gasoline engines. Emission control systems are widely utilized to reduce the amount of these pollutants emitted to atmosphere, and typically achieve very high efficiencies once they reach their operating temperature (typically, 200° C. and higher). However, these systems are relatively inefficient below their operating temperature (the “cold start” period).
One exhaust gas treatment component utilized to clean exhaust gas is the NOx adsorber catalyst (or “NOx trap”). NOx adsorber catalysts are devices that adsorb NOx under lean exhaust conditions, release the adsorbed NOx under rich conditions, and reduce the released NOx to form N2. A NOx adsorber catalyst typically includes a NOx adsorbent for the storage of NOx and an oxidation/reduction catalyst.
The NOx adsorbent component is typically an alkaline earth metal, an alkali metal, a rare earth metal, or combinations thereof. These metals are typically found in the form of oxides. The oxidation/reduction catalyst is typically one or more noble metals, preferably platinum, palladium, and/or rhodium. Typically, platinum is included to perform the oxidation function and rhodium is included to perform the reduction function. The oxidation/reduction catalyst and the NOx adsorbent are typically loaded on a support material such as an inorganic oxide for use in the exhaust system.
The NOx adsorber catalyst performs three functions. First, nitric oxide reacts with oxygen to produce NO2 in the presence of the oxidation catalyst. Second, the NO2 is adsorbed by the NOx adsorbent in the form of an inorganic nitrate (for example, BaO or BaCO3 is converted to Ba(NO3)2 on the NOx adsorbent). Lastly, when the engine runs under rich conditions, the stored inorganic nitrates decompose to form NO or NO2 which are then reduced to form N2 by reaction with carbon monoxide, hydrogen and/or hydrocarbons (or via NHx or NCO intermediates) in the presence of the reduction catalyst. Typically, the nitrogen oxides are converted to nitrogen, carbon dioxide and water in the presence of heat, carbon monoxide and hydrocarbons in the exhaust stream.
PCT Intl. Appl. WO 2004/076829 discloses an exhaust-gas purification system which includes a NOx storage catalyst arranged upstream of an SCR catalyst. The NOx storage catalyst includes at least one alkali, alkaline earth, or rare earth metal which is coated or activated with at least one platinum group metal (Pt, Pd, Rh, or Ir). A particularly preferred NOx storage catalyst is taught to include cerium oxide coated with platinum and additionally platinum as an oxidizing catalyst on a support based on aluminium oxide. EP 1027919 discloses a NOx adsorbent material that comprises a porous support material, such as alumina, zeolite, zirconia, titania, and/or lanthana, and at least 0.1 wt % precious metal (Pt, Pd, and/or Rh). Platinum carried on alumina is exemplified.
In addition, U.S. Pat. Nos. 5,656,244 and 5,800,793 describe systems combining a NOx storage/release catalyst with a three way catalyst. The NOx adsorbent is taught to comprise oxides of chromium, copper, nickel, manganese, molybdenum, or cobalt, in addition to other metals, which are supported on alumina, mullite, cordierite, or silicon carbide.
PCT Intl. Appl. WO 2009/158453 describes a lean NOx trap catalyst comprising at least one layer containing NOx trapping components, such as alkaline earth elements, and another layer containing ceria and substantially free of alkaline earth elements. This configuration is intended to improve the low temperature, e.g. less than about 250° C., performance of the LNT.
US 2015/0336085 describes a nitrogen oxide storage catalyst composed of at least two catalytically active coatings on a support body. The lower coating contains cerium oxide and platinum and/or palladium. The upper coating, which is disposed above the lower coating, contains an alkaline earth metal compound, a mixed oxide, and platinum and palladium. The nitrogen oxide storage catalyst is said to be particularly suitable for the conversion of NOx in exhaust gases from a lean burn engine, e.g. a diesel engine, at temperatures of between 200 and 500° C.
Conventional lean NOx trap catalysts often have significantly different activity levels between activated and deactivated states. This can lead to inconsistent performance of the catalyst, both over the lifetime of the catalyst and in response to short term changes in exhaust gas composition. This presents challenges for engine calibration, and can cause poorer emissions profiles as a result of the changing performance of the catalyst.
As with any automotive system and process, it is desirable to attain still further improvements in exhaust gas treatment systems. We have discovered a new NOx adsorber catalyst composition with improved NOx storage and conversion characteristics, as well as improved CO conversion. It has surprisingly been found that these improved catalyst characteristics are observed in both the active and deactivated states.